Weakly collisional , magnetised plasmas characterised by anisotropic viscosity and conduction are ubiquitous in galaxies , halos and the intracluster medium ( ICM ) .
Cosmic rays ( CRs ) play an important role in these environments as well , by providing additional pressure and heating to the thermal plasma .
We carry out a linear stability analysis of weakly collisional plasmas with cosmic rays using Braginskii MHD for the thermal gas .
We assume that the CRs stream at the Alfvén speed , which in a weakly collisional plasma depends on the pressure anisotropy ( \Delta p ) of the thermal plasma .
We find that this \Delta p -dependence introduces a phase shift between the CR-pressure and gas-density fluctuations .
This drives a fast-growing acoustic instability : CRs offset the damping of acoustic waves by anisotropic viscosity and give rise to wave growth when the ratio of CR pressure to gas pressure is \gtrsim \alpha \beta ^ { -1 / 2 } , where \beta is the ratio of thermal to magnetic pressure , and \alpha , typically \lesssim 1 , depends on other dimensionless parameters .
In high- \beta environments like the ICM , this condition is satisfied for small CR pressures .
We speculate that the instability studied here may contribute to the scattering of high-energy CRs and to the excitation of sound waves in galaxy-halo , group and cluster plasmas , including the long-wavelength X-ray fluctuations in Chandra observations of the Perseus cluster .
It may also be important in the vicinity of shocks in dilute plasmas ( e.g. , cluster virial shocks or galactic wind termination shocks ) , where the CR pressure is locally enhanced .